Very bad performance on Cycles OpenCL using Ubuntu 18.04 and Radeon VII
Have you tried updating Ubuntu to 18.04.2 with HWE (4.18 Kernel) and AMD's most recent 19.20 driver?

A few months ago I had issues getting my Dual RX Vega64 setup working (on a Ryzen 1700 system with Ubuntu 18.04): Just a single GPU worked fine, but when I installed both GPUs, Blender refused to use any of them for OpenCL rendering with Cycles. 

Some debugging and writing small test programs later, I figured out the AMD's driver refused to open an OpenCL-context on the Dual-GPU-setup when already an OpenGL-context had been opened before. (With just a single GPU installed this worked fine, though.)

Since then I updated my Ubuntu to 18.04.2 with HWE (Linux 4.18 kernel, instead of 4.15 without the HWE) and updated my driver to the new 19.20 version, and now my Dual-GPU-setup works fine out-of-the-box as if nothing ever happened (with rendering speed-up of 50-100% compared to my single-GPU-setup).